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          <th colspan="3" align="center">Reading and Writing Database Records</th>
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          <td width="20%" align="left"><a accesskey="p" href="DBEntry.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 8. Database Records</th>
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="usingDbt"></a>Reading and Writing Database Records</h2>
          </div>
        </div>
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      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="usingDbt.html#databaseWrite">Writing Records to the Database</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="usingDbt.html#databaseRead">Getting Records from the Database</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="usingDbt.html#recordDelete">Deleting Records</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="usingDbt.html#datapersist">Data Persistence</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        When reading and writing database records, be aware that there are some
        slight differences in behavior depending on whether your database supports duplicate
        records. Two or more database records are considered to be duplicates of 
        one another if they share the same key. The collection of records
        sharing the same key are called a <span class="emphasis"><em>duplicates set.</em></span>

        <span>
            In DB, a given key is stored only once for a single duplicates set.
        </span> 
    </p>
      <p>
        By default, DB databases do
        not support duplicate records. Where duplicate records are supported,
        cursors (see below) are <span>typically</span> used
        to access all of the records in the duplicates set.
    </p>
      <p>
        DB provides two basic mechanisms for the storage and retrieval of database
        key/data pairs:
    </p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>
            The 
            <code class="methodname">Database.put()</code> 
             
             
            and
            <code class="methodname">Database.get()</code> 
             
             
            methods provide the easiest access for all non-duplicate records in the database. 
            These methods are described in this section.
        </p>
          </li>
          <li>
            <p>Cursors provide several methods for putting and getting database
        records. Cursors and their database access methods are described in
        <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.</p>
          </li>
        </ul>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="databaseWrite"></a>Writing Records to the Database</h3>
            </div>
          </div>
        </div>
        <p>
        Records are stored in the database using whatever organization is
        required by the access method that you have selected. In some cases (such as
        BTree), records are stored in a sort order that you may want to define
        (see <a class="xref" href="btree.html#comparators" title="Setting Comparison Functions">Setting Comparison Functions</a> for more information). 
      </p>
        <p>
        In any case, the mechanics of putting and getting database records do not
        change once you have selected your access method, configured your
        sorting routines (if any), and opened your database. From your
        code's perspective, a simple database put and get is largely the
        same no matter what access method you are using.
      </p>
        <p>You can use the following methods to put database records:</p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                <code class="methodname">Database.put()</code>
              </p>
              <p>
            Puts a database record into the database. If your database does not
            support duplicate records, and if the provided key already exists in
            the database, then the currently existing record is replaced with
            the new data.
          </p>
            </li>
            <li>
              <p>
                <code class="methodname">Database.putNoOverwrite()</code>
              </p>
              <p>
            Disallows overwriting (replacing) an existing record in the
            database. If the provided key already exists in the database, 
            then this method returns 
            <code class="literal">OperationStatus.KEYEXIST</code> even if
            the database supports duplicates.
          </p>
            </li>
            <li>
              <p>
                <code class="methodname">Database.putNoDupData()</code>
              </p>
              <p>
            Puts a database record into the database. If the provided key
            and data already exists in the database (that is, if you are
            attempting to put a record that compares equally to an existing
            record), then this returns
            <code class="literal">OperationStatus.KEYEXIST</code>.
          </p>
            </li>
          </ul>
        </div>
        <p>
        When you put database records, you provide both the key and the data as
        <code class="classname">DatabaseEntry</code> objects. This means you must
        convert your key and data into a Java <code class="literal">byte</code> array. For
        example:
      </p>
        <a id="java_dbt3"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;

...

// Database opens omitted for clarity.
// Databases must NOT be opened read-only.

String aKey = "myFirstKey";
String aData = "myFirstData";

try {
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
    myDatabase.put(null, theKey, theData);
} catch (Exception e) {
    // Exception handling goes here
} </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="databaseRead"></a>Getting Records from the Database</h3>
            </div>
          </div>
        </div>
        <p>
        The <code class="classname">Database</code> class provides several
        methods that you can use to retrieve database records. Note that if your
        database supports duplicate records, then these methods will only ever
        return the first record in a duplicate set. For this reason, if your
        database supports duplicates, you should use a cursor to retrieve
        records from it. Cursors are described in <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.
      </p>
        <p>
          You can use either of the following methods to retrieve records from the database:
      </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                <code class="methodname">Database.get()</code>
              </p>
              <p>Retrieves the record whose key matches the key provided to the
          method. If no records exists that uses the provided key, then
          <code class="literal">OperationStatus.NOTFOUND</code> is returned.</p>
            </li>
            <li>
              <p>
                <code class="methodname">Database.getSearchBoth()</code>
              </p>
              <p>Retrieve the record whose key matches both the key and the data
          provided to the method. If no record exists that uses the provided
          key and data, then <code class="literal">OperationStatus.NOTFOUND</code> is
          returned.</p>
            </li>
          </ul>
        </div>
        <p>Both the key and data for a database record are returned as
      byte arrays  in <code class="classname">DatabaseEntry</code> objects. These objects are
      passed as parameter values to the <code class="methodname">Database.get()</code> method.
      </p>
        <p>In order to retrieve your data once <code class="classname">Database.get()</code>
      has completed, you must retrieve the <code class="literal">byte</code> array stored 
      in the <code class="classname">DatabaseEntry</code> and then convert that 
      <code class="literal">byte</code> array back to the
      appropriate datatype. For example:</p>
        <a id="java_dbt4"></a>
        <pre class="programlisting">package db.GettingStarted;
      
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;

...

Database myDatabase = null;
// Database opens omitted for clarity.
// Database may be opened read-only.  
  
String aKey = "myFirstKey";

try {
    // Create a pair of DatabaseEntry objects. theKey
    // is used to perform the search. theData is used
    // to store the data returned by the get() operation.
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    DatabaseEntry theData = new DatabaseEntry();
    
    // Perform the get.
    if (myDatabase.get(null, theKey, theData, LockMode.DEFAULT) ==
        OperationStatus.SUCCESS) {

        // Recreate the data String.
        byte[] retData = theData.getData();
        String foundData = new String(retData, "UTF-8");
        System.out.println("For key: '" + aKey + "' found data: '" + 
                            foundData + "'.");
    } else {
        System.out.println("No record found for key '" + aKey + "'.");
    } 
} catch (Exception e) {
    // Exception handling goes here
}</pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="recordDelete"></a>Deleting Records</h3>
            </div>
          </div>
        </div>
        <p>

        You can use the 
            <code class="methodname">Database.delete()</code>
            
            
        method to delete a record from the database. If your database supports
        duplicate records, then all records associated with the provided key are
        deleted. To delete just one record from a list of duplicates, use a
        cursor. Cursors are described in <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.

      </p>
        <p>
          You can also delete every record in the database by using
              <code class="methodname">Environment.truncateDatabase().</code>
              
              
        </p>
        <p>For example:</p>
        <a id="java_dbt5"></a>
        <pre class="programlisting">package db.GettingStarted;

import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.Database;

...

Database myDatabase = null;
// Database opens omitted for clarity.
// Database can NOT be opened read-only.  
  
try {
    String aKey = "myFirstKey";
    DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
    
    // Perform the deletion. All records that use this key are
    // deleted.
    myDatabase.delete(null, theKey); 
} catch (Exception e) {
    // Exception handling goes here
}</pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="datapersist"></a>Data Persistence</h3>
            </div>
          </div>
        </div>
        <p>
            When you perform a database modification, your modification is made
            in the in-memory cache.  This means that your data modifications
            are not necessarily flushed to disk, and so your data may not appear
            in the database after an application restart.
        </p>
        <p>
            Note that as a normal part of closing a database, its cache is
            written to disk. However, in the event of an application or system
            failure, there is no guarantee that your databases will close
            cleanly. In this event, it is possible for you to lose data. Under
            extremely rare circumstances, it is also possible for you to
            experience database corruption.
        </p>
        <p>
            Therefore, if you care if your data is durable across system
            failures, and to guard against the rare possibility of
            database corruption, you should use transactions to protect your
            database modifications. Every time you commit a transaction, DB
            ensures that the data will not be lost due to application or 
            system failure.  Transaction usage is described in the
                

                

                <span>
                <em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em> guide.
                </span>
        </p>
        <p>
            If you do not want to use transactions, then the assumption is that
            your data is of a nature that it need not exist the next time your
            application starts. You may want this if, for example, you are using
            DB to cache data relevant only to the current application
            runtime.
        </p>
        <p>
            If, however, you are not using transactions for some reason and you
            still want some guarantee that your database modifications are
            persistent, then you should periodically
                <span>run environment syncs.</span>
                
                
            Syncs cause any dirty entries in the in-memory cache and the
            operating system's file cache to be written to disk. As
            such, they are quite expensive and you should use them sparingly.
        </p>
        <p>
            Remember that by default a sync is performed any time a non-transactional
            database is closed cleanly. (You can override this behavior by
            specifying 
                 
                <code class="literal">true</code> 
            on the call to 
                
                
                <span><code class="methodname">Database.close()</code>.)</span>
            
            That said, you can manually run a sync by calling
            
                
                
                <code class="methodname">Database.sync().</code>
            
        </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
                If your application or system crashes and you are not using
                transactions, then you should either discard and recreate your 
                databases, or verify them. You can verify a database using
                    
                    
                    <span>Database.verify().</span>
                If your databases do not verify cleanly, use the 
                <span class="command"><strong>db_dump</strong></span> command to salvage as much of the
                database as is possible. Use either the <code class="literal">-R</code> or
                <code class="literal">-r</code> command line options to control how
                aggressive <span class="command"><strong>db_dump</strong></span> should be when salvaging
                your databases.
            </p>
        </div>
      </div>
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